What periodic trend is observed in ionization energy as you move from top to bottom in a group?

As you move from top to bottom within a group of the periodic table, ionization energy generally decreases. This trend can be attributed to several key factors.

Firstly, as you go down a group, the number of electron shells increases. Each additional shell means that the outermost electrons are farther away from the nucleus. This increased distance reduces the effective nuclear charge that the outer electrons experience because there are more inner shell electrons between the nucleus and the outermost electrons.

Secondly, electron shielding comes into play, as inner shell electrons repel outer shell electrons, further diminishing the attractive force from the nucleus. This shielding effect makes it easier for the outermost electron to be removed, leading to a lower ionization energy.

Consequently, it requires less energy to remove an electron from an atom as one moves down a group due to this increased distance and shielding effect. The combination of these factors explains why ionization energy decreases from top to bottom in a group on the periodic table.